In the fabrication of integrated circuitry, numerous devices are packed into a small area of a semiconductor substrate to create an integrated circuit. Many of the individual devices are electrically isolated from one another. Accordingly, electrical isolation is an integral part of semiconductor device design for preventing unwanted electrical coupling between adjacent components and devices.
As the size of integrated circuits is reduced, the devices that make up the circuits are positioned closer together. One method of isolating circuit components is trench isolation. Such occurs by etching trenches into semiconductive material of a substrate and filling the trenches with a suitable electrical isolation material, typically insulative material although other materials such as a semiconductive material to which a potential can be applied might also be used. As the density of components on the semiconductor substrate has increased, the widths of the trenches have decreased. Further, the depths of the trenches have tended to increase.
The continual reduction in device or feature sizes places ever greater demands on the techniques used to form the features, such as isolation trenches and the active or circuitry area there-between. For example, photolithography is commonly used to pattern the features. A concept commonly referred to as “pitch” can be used to describe the size of repeating features, such as a series of trench lines. Pitch is defined as the distance between an identical point in two neighboring features. In the context of isolation trenches, the trench is essentially defined by the cross-sectional dimension of the trench and the immediately adjacent space between two adjacent trenches. As a result, pitch can be viewed as the sum of the width of the trench and the width of the space on one side of the trench separating that trench from a neighboring trench. However, due to factors such as optics and light or radiation wave length, photolithography techniques have a minimum pitch below which a particular photolithographic technique cannot reliable form features. Thus, the minimum pitch of a photolithographic technique is an obstacle to continued feature size reduction.
“Pitch doubling” or “pitch multiplication” are techniques proposed for extending the capabilities of photolithographic process beyond their minimum pitch. Such techniques typically fabricate features in a mask to a minimum capable dimension using photolithography. Then, the widths of the mask features are expanded, for example by sidewall oxidation or deposition and anisotropic etch to form spacers such that the width increase on at least one side of the feature is of a dimension less than the minimum photolithographic feature resolution. The original mask features are then etched selectively relative to the added material, thus leaving a mask having feature widths which are less than that which can be achieved using photolithography alone.
A particular problem which motivated some embodiments of the invention is described with reference to FIG. 1. Such depicts a semiconductor substrate 10 comprising bulk semiconductive material 11 having a mask 12 that has been formed thereover. Mask 12 comprises, in the depicted cross-section, parallel feature lines 13 comprised of a first masking material 14 received over a second masking material 15 which is received on semiconductor substrate 11. An example material 15 is pad oxide, while an example material 14 is silicon nitride. Mask 12 has been utilized to form isolation trenches 16 within bulk semiconductor material 11. Silicon dioxide 17, as an isolation material, has been deposited over mask 12 to within isolation trenches 16. Unfortunately as shown, some of the features 13 of mask 12 have toppled or leaned laterally in the direction of the isolation trenches. This occurred during the fill of trenches 16 with material 17 likely due to stresses introduced during the fill. This can also lead to severe bending of the isolation structures and/or including the material 11 immediately beneath mask 12, for example as shown. This particular problem was found to manifest as the width of the isolation trenches or the width of the material between the isolation trenches fell to 35 nanometers and below. Such might occur regardless of whether pitch multiplication techniques are used.